Differences in speed is what is dangerous. On a 65mph highway, one guy doing 90 while everyone else is doing 65 is a problem. On a theoretical road with a speed limit of 100mph, all of those Toyota Tercels that can only go a max of 65mph -- that also is a problem.

Generally, what I've noticed here in the Bay area, if you are going with the flow of traffic, whatever speed that is, the cops will not mess with you. If you are changing lanes and darting between cars, then you will get your share of attention.